1. Field of the Invention
The present invention relates to flexible superconducting compound wires, such as, stranded wires, braided wires, and transposed wires, composed of a plurality of strands, each having a layer of a superconducting compound and relates to a method for manufacturing thereof.
2. Description of the Prior Art
Various extensive studies have been made on superconducting compounds as wire materials for high magnetic field magnets, since they have very high critical magnetic field (Hc.sub.2) as compared with superconducting alloys. Typical superconducting compounds include .beta.-W type compound such as Nb.sub.3 Sn, Nb.sub.3 Al, Nb.sub.3 Ga, Nb.sub.3 (AlGc), V.sub.3 Ga, V.sub.3 Si; NaCl type compounds, such, as NbN, (NbTi)N, NbC; and Laves phase compounds, such as, V.sub.2 Hf, V.sub.2 Zr. Among them only Nb.sub.3 Sn and V.sub.3 Ga are at present in practical use.
Processing of the superconducting compounds into wires have been always accompanied with the problem of mechanical brittleness and various wire manufacturing methods have been developed and proposed for solving the problem up to now. The representative methods include a composite method (for example, a composite of niobium and tin is subjected to roll processing or wire drawing and then subjected to heat treatment); a surface diffusion method (molten tin or garium is coated on vanadium or niobium base and the coated metal base is subjected to a diffusion heat treatment) and a vapour-phase reduction method (gaseous halide of component elements of a superconducting compound is reduced by hydrogen to form the compound on the base). Other methods such as a reactive spattering method and a plasma spray method may be mentioned.
Wire coils of superconducting compound produced by these conventional methods have been in practical use only in the form of tape.
The conventional superconducting compound in the tape shape, however, have the following defects which can not be avoided essentially.
The first defect is that the tape shaped superconductor cannot be coiled around the magnet bobbin in solenoidal winding so that the magnet must be made by arranging a required number of Baumkuchen-like coils or pan-cake type coils.
Secondarily, there is a problem of the structural anisotropy causing anisotropy in the critical current and, as a result of the coil performance. The second defect is that the so-called pinning force of the magnetic flux lowers in the magnetic field direction perpendicular to the tape surface, that is, at the end portions of the magnet so that instability is caused.
Thirdly, there is a problem of irregularity in the magnetic field when the tape is used as a magnet coil. In case the magnetic field intensity changes along the width of the tape, the current flows more to portions of the weak magnetic field so that the current density is not uniform, thus causing the disorder of the magnetic field distribution. Further, there are always spaces between the pan-cake type coils, and disorder of the magnetic field distribution is induced by these spaces. Thus, a highly uniform magnet cannot be made from the tape shaped coils.
As a fourth defect, the total packing factor can never be increased. The total current density of the magnet as a whole lowers if the coiled wires have the same current density because a magnet of pan-cake type has a low packing factor as mentioned before. Further, in the case of a pan-cake type coil, the number of coils to form a magnet is small so that each coil must be of a large current type. As a result, it is necessary to provide a large amount of stabilizing materials which, in turn, causes lowering of the average current density.
As a fifth defect, the tape shaped superconductor has difficulty in respect of compatibility with the conventional NbTi alloy wire. Thus, from point of economic design of the magnet, it is most preferable to use superconducting compound wires, stable in a high magnetic field, for the core portion of the magnet which is the high magnetic field zone, and use inexpensive NbTi wire for the outer, middle and low magnetic field zones. However, the conventional tape shaped superconductor must be in the form of a pan-cake type coil so that there are difficulties in respect of the structural compatibility of both coils. Further, the tape shaped superconductor must be of a large current type as mentioned above so that it is difficult to obtain the current matching of both types of the coils.
Therefore, an object of the present invention is to provide superconducting compound wires having excellent flexibility which can be handled just as superconducting alloy wires.
Another object of the present invention is to provide a superconducting stranded, woven and transposed compound wires having excellent flexibility.
Still another object of the present invention is to provide flexible superconducting compound wires suitable for use in a magnet.
Still another object of the present invention is to provide a method for manufacturing the above superconducting compound wires having excellent flexibility.
According to the present invention, there are provided superconducting compound wires wherein some or all of the plural strands having a continuous layer of superconducting compound are assembled in such a manner as to have a certain inclination to the lengthwise direction of the superconducting compound wires, and the strands do not substantially adhere to each other.
The present invention also provides a method for the manufacture of superconducting compound wires, which comprises assembling together some or all of strands having a compound layer formed by a heat treatment, in such a manner that they have a certain inclination to the lengthwise direction of the superconducting compound wires, heating the assembled strands to a temperature for forming the compound, and cooling the strands.
The wires in which the strands are assembled together so as to have an inclination to the lengthwise direction of the wires include stranded wires, woven wires, and also transposed wires, any other wires in which not all of the component strands are assembled in parallel to the lengthwise direction of the wire.